The present invention relates to a method for soldering a component to a printed circuit board
In mounting a component to a mounting surface of a printed circuit board by soldering, a preliminary solder is applied to the component to prevent non-wetting. With this preliminary soldered component put close to or abutted on the mounting surface of a mounting place, melted solder is supplied to the place for final soldering. In this case, the composition of the solder preliminary applied to the component is the same as that used for the final soldering.
JP-A7-254780 discloses a technology aimed to improve bonding strength. According to the technology, as shown in FIG. 6A, a preliminary solder 3 is applied to a mounting surface 2 made of Cu, of a printed circuit board 1 by using a high temperature solder (Sn-Pb) having a smaller tin content than an eutectic solder does. Then, as shown in FIG. 6B, the eutectic solder 4 is applied to the preliminarily soldered surface and reflow-processing is performed by blowing warm air to the eutectic solder 4. Then, as shown in FIG. 6C, the eutectic solder 4 is melted and solidified around an electrode 5a of a chip component 5 to finally solder the chip component 5.
When the solder 3 is preliminarily applied as shown in FIG. 6, wetting of the eutectic solder 4 to the solder 3 preliminarily applied is better than that of the eutectic solder 4 to the mounting surface 2 made of Cu. Also, the preliminary soldering makes a bonded interface stronger than that which has undergone no preliminary soldering. In FIG. 6C, the strong bonded interface is indicated by a symbol J1.
Though the configuration as shown in the FIG. 6 provides some improvements in the bonding strength, a higher bonding strength is required at present.
Particularly, in view of recycling of home electronic appliances, higher bonding strength is required of soldering using a lead-free solder which is attracted considerable attention as a solder material for use in home electronic appliances.
It is an object of the present invention to provide a soldering method aimed to obtain a higher bonding strength than conventionally obtained ones.
The soldering method of the present invention is characterized in that in soldering a component to a mounting surface which has been preliminarily soldered, a solder for preliminary soldering has a higher melting point than a solder for final soldering and comprises elements of the mounting surface and of at least part of the solder for final soldering, and the component is soldered to the preliminarily soldered mounting surface with the solder for final soldering.
Further, the soldering method of the present invention is characterized in that the mounting surface is made of Cu, the elements of the solder for final soldering are any one of (Sn-Ag-Cu-Bi), (Sn-Ag system plus additives), (Sn-Zn system plus additives), (Sn-Bi system plus additives) and (Sn-In system plus additives), and the element of the solder for preliminary soldering is Sn-Cu.
Further, the soldering method of the present invention is characterized in that in soldering a component which has been preliminarily soldered to a mounting surface, a solder for preliminary application had a higher melting point than a solder for final soldering of the component, and comprises elements of a to-be-soldered portion of the component and of at least a part of components of the solder for final soldering, and the preliminarily soldered component is soldered to the mounting surface with the solder for final soldering.
Further, the soldering method of the present invention is characterized in that the element of the to-be-soldered portion of the component is Cu, the elements of the solder for final soldering are any one selected from (Sn-Ag-Cu-Bi), (Sn-Ag system plus additives), (Sn-Zn system plus additives), (Sn-Bi system plus additives) and (Sn-In system plus additives), and the element of solder for preliminary soldering is Sn-Cu.
Still further, the soldering method of the present invention is characterized in that in soldering a component to a mounting surface, the mounting surface of Cu is preliminarily soldered with a solder comprising an Sn-Cu element, a to-be-soldered portion of Cu of the component is preliminarily soldered with the solder having the Sn-Cu element, and the preliminarily soldered component is soldered to the preliminary soldered mounting surface with a solder for final soldering, said solder for final soldering having elements of any one selected from (Sn-Ag-Cu-Bi), (Sn-Ag system plus additives), (Sn-Zn system plus additives), (Sn-Bi system plus additives) and (Sn-In system plus additives) for final soldering to the mounting surface preliminarily soldered.
As described above, according to the soldering method of the present invention, in soldering a component to a preliminarily soldered mounting surface, a solder used for such preliminary soldering has a higher melting point than a solder for final soldering of the component and comprises elements of a to-be-soldered portion of the component and of at least part of the solder for final soldering, and the preliminarily soldered component is soldered to the mounting surface with the solder for final soldering. Therefore, higher bonding strength is obtained not only in the bonding interface between the solder for preliminary soldering and the solder for final soldering, but also in the bonding interface between the mounting surface and the solder for preliminary soldering.
When the mounting surface is Cu, the solder for final soldering comprises any one selected from (Sn-Ag-Cu-Bi), (Sn-Ag system plus additives), (Sn-Zn system plus additives), (Sn-Bi system plus additives) and (Sn-In system plus additives), and the solder for preliminary soldering is Sn-Cu, higher bonding strength can be obtained also from such soldering as using a lead-free solder.